WO 2000/32259 discloses an injection device for injecting a medicament. It is noted that the prior art injection device comprises two injectors for injecting first a numbing agent with a first needle, and then the medicated liquid with a second needle, which inserted deeper into the tissue. The injectors have a similar construction though. In order to simplify the description of the prior art injection device, it will be describe from a one injector perspective. Thus, for each injector, the injection device primarily comprises two main parts, a cartridge containing a liquid and having a needle, and an operation part having a cartridge holder, a plunger rod, connectable to a plunger of the cartridge, and a drive device for driving the plunger rod within the liquid container of the cartridge, such that the liquid is expelled from the container through an opening of a needle provided at a front end of the container. The cartridge is disposable, i.e. it is exchangeable for a new cartridge after use. More particularly, the plunger comprises a releasable snap lock for the plunger rod, as follows. The plunger comprises a cup-shaped receptacle for receiving a ball-shaped end of the plunger rod. The mouth of the receptacle is partly covered by radially inwardly extending finger projections, which define a circular opening having a smaller diameter than the ball-shaped end. Since the finger projections are a bit flexible, the ball-shaped end can be forced passed them into the receptacle, and be withdrawn out of the receptacle, while still being retained in the receptacle during operation of the plunger. Thereby, in short the injection device is operated as follows. The cartridge is mounted at the cartridge holder of the operation part, the injection device is activated to move the plunger rod forward into abutment against the finger projections of the receptacle. By moving the plunger rod further forwards, the container is moved forwards, and thus the needle is inserted. When the needle is fully inserted, the plunger rod, by further forward motion thereof, enters the receptacle passed the finger projections. Then the liquid is expelled by driving the plunger rod to move further forwards. When the plunger rod is moved rearwards again, the plunger is retracted to a rear stop, then the needle is retracted, and, finally, the plunger rod leaves the receptacle, and the cartridge can be removed.
The above-described known injection device may work well for one time injections. However, another large application area for injection devices is multiple injections with the same cartridge, such as for different skin treatments, where a sub-amount of the liquid contained in the cartridge is injected at each injection. However, then a problem is encountered when pulling out the needle for the next injection. Due to the viscosity of the liquid, the needle is likely to drool when it is pulled out.